The invention relates to the identification and quantification of chemical properties. In particular, the invention relates to a method and apparatus for identifying and quantifying chemical species using transducers.
A number of analysis techniques are known by which various chemicals can be quantified and identified. For example coulometric, titrimetric, or calorimetric analyses techniques are known. Each of these techniques requires extended procedures that render near real-time and real-time measurements impractical. Infrared absorption spectroscopy and indirect chemiluminescence are known techniques having better response time. However, indirect chemiluminescence techniques have limited accuracy and infrared absorption spectroscopy techniques are relatively expensive to implement. Further, electrochemical transducers are well known for use in amperometric, coulometric, and potentiometric systems. However, such single transducers do not exhibit good chemical specificity, often require considerable maintenance and are particularly expensive. Other techniques rely upon significantly less expensive high temperature semiconductor transducers. However, these transducers also have a low chemical specificity and often have extremely demanding electrical power requirements.
It is known that a resonator, such as a piezoelectric quartz crystal, may be used as a chemical measurement transducer. Such transducers are generally more sensitive and selective and less expensive than other transducers. Resonators are inexpensive, readily available and can be coated with a compound, for example, a sensing film, responsive specifically to the presence of the chemical to be detected to control their chemical specificity. In particular, each resonator has a particular frequency of resonation when an electric potential is applied across it. As certain species are deposited in the surface of the resonator, the natural frequency of vibration changes. The changing frequency can be compared to that of a reference resonator not exposed to the chemical. The film can be used to permit specific species to pass therethrough to be deposited on the resonator surface. Accordingly, the transducer can be tuned to be responsive to various chemicals or chemical species in a known manner. However, the transducer response is not unique for each chemical or group of chemicals and thus known transducers based on resonators are not always accurate or even useful for all chemical species.
A single sensor may exhibit non-specific response in some sensing applications. Thus, identification and quantification of a target species may be adversely influenced. To overcome this possible adverse influence, arrays of sensors may be provided, in which at least one of the sensors in the array comprise a chemical sensor. Sensor arrays permit pattern recognition from the data collected that reflects the nature, property, and quantity of the target species. The number of sensors in a sensor array may vary; for example, the number may be two sensors to thousands of sensors, in which the number of sensors is usually dependent on various application criteria. These application include, but not limited to, type of desired sensor response, complexity of analyzed mixture, concentration of vapor or target species, signal levels produced by each sensor, noise levels produced by each sensor, similarity of response patterns, combinations thereof, and other sensor-related factors.
It is also known to reduce the number of transducers in transducer arrays by measuring plural parameters from a single sensing element. For example, U.S. Pat. No. 5,076,094 discloses a method of identification and quantification of chemical species in which changes in both the velocity and the attenuation of an acoustic wave traveling through a thin film into which the chemical species is sorbed are measured. The dual output response provides two independent transducer responses from a single sensing device thereby providing twice as much information as a single output transducer and allowing a single transducer to provide both the concentration and the identity of a chemical species. It is also known to combine optical detection with acoustic wave measurements by coating an oscillating quartz crystal with a fluorescent dye and measuring both fluorescence intensity and the fundamental oscillation frequency as disclosed in U.S. Pat. No. 5,411,709.
Other types of optical spectroscopic measurements also have been combined with acoustic wave measurements. For example, it is known to combine surface acoustic wave (SAW) transducer measurements with direct in situ Fourier transform infrared external-reflectance spectroscopy as disclosed in Effective Use of Molecular Recognition in Gas Sensing; Hierlemann, A., Ricco, A. J., Bodenhofer, K. and Gopel, W.; Anal. Chem.; 1999,71, 3022-3035 and Reflectance Infrared Spectroscopy on Operating Surface Acoustic Wave Chemical Sensors During Exposure to Gas-Phase Analytes; Thomas, R. C., Hierlemann, A., Staton, A. W., Hill, M., and Ricco, A. J.; Anal. Chem.; 1999, 71, 3615-3621.
Finally, it is known to conduct simultaneous electrical conductivity and piezoelectric mass measurements on iodine-doped phthalocyanine Langmuir-Blodgett films to reduce the number of sensing elements by combining two measurement techniques on a single sensing element as disclosed in Simultaneous Electrical Conductivity and Piezoelectric Mass Measurements on Iodine-Doped Phthalocyanine Langmuir-Blodgett Films; Langmuir 1986, 2, 513-519.
Therefore, a need exists to provide enhanced apparatus for measurement of dual responses from a single sensing device that can provide more information compared to a single output transducer.
A first aspect of the invention comprises an apparatus for determining chemical properties of an analyte. The apparatus comprises a vessel divided into plural compartments, a first resonator comprising a first side coated with a first sensing film and a second side coated with a second sensing film, the first side of the first resonator being exposed to a different one of the compartments than the second side of the first resonator. An electric power source is coupled to the first resonator and adapted to place an electric potential between the first side and the second side of the first resonator, and a frequency detector is coupled to the first resonator and adapted to detect the frequency of resonation of the first resonator.
A second aspect of the invention comprises a method for determining chemical composition. The method comprises a first step of exposing a first side of a first resonator coated with a first sensing film to an analyte and exposing a second side of the first resonator coated with a second sensing film to a blank, a first step of measuring the fundamental frequency of the resonator during the first step of exposing, a second exposing step comprising exposing the second side of the first resonator to the analyte and exposing the first side of the first resonator to the blank, a second step of measuring the fundamental frequency of the resonator during the second exposing step, and determining the chemical properties of the analyte based on the results of the first step of measuring and the second step of measuring.
A third aspect of the invention comprises an apparatus for determining chemical composition. The apparatus comprises first means for measuring the fundamental frequency of a first resonator while a first side of the first resonator coated with a first sensing film is exposed to an analyte and a second side of the first resonator coated with a second sensing film is exposed to a blank, second means for measuring the fundamental frequency of the resonator while the second side of the first resonator is exposed to the analyte and the first side of the first resonator is exposed to the blank, and means for determining the chemical properties of the analyte based on the results obtained by said first means for measuring and said second means for measuring.
A fourth aspect of the invention comprises an apparatus for determining chemical properties of an analyte. The apparatus comprises a vessel divided into plural compartments, a first quartz crystal microbalance resonator having a first side coated with a first sensing film and a second side coated with a second sensing film, the first side of the first quartz crystal microbalance resonator being exposed to a different one of the plural compartments than the second side of the first quartz crystal microbalance resonator, an electric power source coupled to the first quartz crystal microbalance resonator and adapted to place an electric potential between the first side and the second side of the first quartz crystal microbalance resonator, and a frequency detector coupled to the first resonator and adapted to detect the frequency of resonation of the first quartz crystal microbalance resonator.